A GaN-based semiconductor laser device having a stacked structure of GaN-based compound semiconductor layers formed on a sapphire substrate or a GaN substrate is evoking much interest as a light-emitting device that emits light in a short wavelength region from an ultraviolet region to green.
The constitution of a GaN-based semiconductor laser device 100 disclosed in JP-A-2000-196201 will be explained below with reference to FIG. 10 showing a schematic partial cross-sectional view of a conventional index-guide type GaN-based semiconductor laser device.
The GaN-based semiconductor laser device 100 disclosed in JP-A-2000-196201 has a stacked structure in which, on a substrate 12 made, for example, of a sapphire substrate having a c-surface as a main surface, a first contacting layer 14 made of n-type GaN, a first cladding layer 16 made of n-type AlGaN, a first light-guiding layer 18 made of n-type InGaN, an active layer 20 having a multiple quantum well structure of GaN/InGaN, a degradation-preventing layer 21 made of AlGaN for preventing the degradation of the active layer 20, a second light-guiding layer 22 made of p-type InGaN, a second cladding layer 24 made of p-type AlGaN and a second contacting layer 26 made of p-type GaN are consecutively stacked. There are many cases where a buffer layer (not shown) made of GaN is grown on the substrate 12 at a low temperature, a substratum layer (not shown) made of GaN is laterally grown on the buffer layer, and then, the first contacting layer 14 is grown. There are also some cases where the first light-guiding layer 18 and the second light-guiding layer 22 are not provided, nor is the degradation-preventing layer 21 provided.
The upper layer 24B of the second cladding layer 24 and the second contacting layer 26 have, for example, a ridge structure extending unidirectionally in the form of a stripe. Further, part of the first contacting layer 14, the first cladding layer 16, the first light-guiding layer 18, the active layer 20, the degradation-preventing layer 21, the second light-guiding layer 22 and the lower layer 24A of the second cladding layer 24 have, for example, a mesa structure extending in the form of a stripe and in the same direction as the extending direction of the ridge structure. That is, the thus-structured GaN-based semiconductor laser device 100 satisfies W′1>W′2 wherein W′1 is a width of the mesa structure and W′2 is a width of the ridge structure.
The ridge structure, the mesa structure and portions of the first contacting layer 14 positioned on both sides of the mesa structure are covered with a protection layer 28 made of SiO2 except for a second opening portion 28A formed on the topmost surface of the ridge structure (i.e., top surface of the second contacting layer 26) and a first opening portion 28B formed on part of the first contacting layer 14. On the second contacting layer 26 positioned in a bottom of the second opening portion 28A, a second electrode 30 having a multi-layered structure of Ti/Au (Ti forms a lower layer and Au forms an upper layer) is provided as an ohmic contact electrode. In the explanation of the multi-layered structure, a material before “/” forms a lower layer and a material after “/” forms an upper layer, and “/” will be used in this sense hereinafter. Further, on the first contacting layer 14 positioned in a bottom of the first opening portion 28B, a first electrode 32 having a multi-layered structure of Ti/Al is provided as an ohmic contact electrode. In addition, provided on the second electrode 30 and the first electrode 32 are a second pad electrode 34 and a first pad electrode 36 that are electrically connected to the second electrode 30 and the first electrode 32, respectively, as leading electrodes. The second pad electrode 34 extends from the second electrode 30 to the top surface of the protection layer 28.
In the above-structured GaN-based semiconductor laser device 100 disclosed in JP-A-2000-196201, the upper layer 24B of the second cladding layer 24 and the second contacting layer 26 have the ridge structure, so that the current passage of electric current injected is limited to decrease the operation current, and that the lateral mode is controlled by means of an effective refractive index difference Δn in a lateral direction. The effective refractive index difference Δn refers to a difference (Δn=nEFF1−nEFF2) between an effective refractive index nEFF1, obtained by measurement along the line A—A in FIG. 10 and an effective refractive index nEFF2 obtained by measurement along the line B—B in FIG. 10.
The above GaN-based semiconductor laser device disclosed in JP-A-2000-196201 has the following problems.
The first problem is that the operation voltage of the GaN-based semiconductor laser device 100 comes to be higher than a desired value or a designed value.
The second problem is as follows. The lateral mode is controlled by means of the effective refractive index difference Δn in a lateral direction. However, it is difficult to increase the thickness of the upper layer 24B of the second cladding layer 24 and it is difficult to decrease the thickness of the lower layer 24A of the second cladding layer 24, so that the effective refractive index difference Δn in a lateral direction is small, and that the stability of the lateral mode is therefore poor. When the upper layer 24B of the second cladding layer 24 is increased in thickness and when the lower layer 24A thereof is decreased in thickness, leak current may flow through the protection layer 28 and the lower layer 24A of the second cladding layer 24.
The third problem is that the process which follows the formation of the stacked structure of GaN-based epitaxial growth layers is complicated and includes many steps, so that it is difficult to improve the productivity. After the formation of the stacked structure, for example, the process requires the steps of forming an etching mask made of SiO2, etching the second contacting layer 26 and further etching an upper portion of the second cladding layer 24 to form the ridge structure; the steps of forming a ZrO2 film on the entire surface and removing the ZrO2 film on the etching mask by removing the etching mask to expose the second contacting layer 26; the steps of forming an etching mask made of SiO2 again on the top surface the ridge structure, etching the lower layer of the second cladding layer and each layer positioned below said layer in the stacked structure to form the mesa structure, further, exposing the first contacting layer 14 and then removing the etching mask; and the step of forming the second electrode 30 on the exposed second contacting layer 26.
JP-A-2000-307184 discloses another method of producing a GaN-based semiconductor laser device. In the second embodiment of the method of producing a GaN-based semiconductor laser device disclosed in the above JP-A-2000-307184, after the formation of a stacked structure of GaN-based epitaxial growth layers, first, the stacked structure is etched to form a mesa structure. Then, a protection layer is formed on the entire surface, an opening portion is formed in the protection layer, an second electrode is formed on the top surface of the second contacting layer positioned in a bottom portion of the opening portion, and then the protection layer is removed. Then, while using the second electrode as an etching mask, the second contacting layer and part of the second cladding layer are etched to form a ridge structure. Then, an insulating layer is formed on the entire surface, and the insulating layer on the second electrode is removed to expose the top surface of the second electrode.
In the above method of producing a GaN-based semiconductor laser device disclosed in JP-A-2000-307184, first, the stacked structure is etched to form the mesa structure. In this case, the top surface of the second contacting layer that is to be a contact surface to the second electrode may be contaminated. Further, there is involved a problem that it is difficult to form a thick insulating layer on both side surfaces of the upper layer of the second cladding layer having the ridge structure.
It is therefore an object of the present invention to provide a nitride-based semiconductor laser device that operates at a low voltage and has excellent lateral mode stability, and a method for producing a nitride-based semiconductor laser device in which the above nitride-based semiconductor laser device can be produced by a process having steps decreased in number.